Process Control Charts Made Simple

What are PBCs?

Process Behavior Charts (PBCs) are powerful visualization tools that help you understand and improve your processes. Originally developed by Walter Shewhart at Bell Laboratories in the 1920s, they are the cornerstone of Statistical Process Control (SPC).

By plotting your data over time, PBCs reveal if changes in your process are normal variations (common cause) or signals (special cause) that require attention. Unlike traditional charts, PBCs use statistical rules to identify significant patterns, helping you make data-driven decisions and avoid overreacting to random fluctuations.

PBCs consist of a time-series chart with three key elements: the centerline (process average), the upper control limit, and the lower control limit. These limits are calculated based on the natural variation in your process, not arbitrary targets.

What is Statistical Process Control?

Statistical Process Control (SPC) is a methodology that uses statistical methods to monitor and control processes. The goal of SPC is to ensure that processes operate efficiently, producing more specification-conforming products with less waste.

SPC helps you distinguish between two types of variation:

• • Common Cause Variation: The natural, expected variation inherent in any process
• • Special Cause Variation: Unexpected changes that signal something unusual is affecting your process

This distinction is crucial - reacting to common cause variation as if it were special often leads to overadjustment and increased variability (known as "tampering"), while ignoring special causes means missing opportunities for improvement.